Procedure for chill casting beryllium composite



United States Patent [72] Inventors William J. Richmond Reading;

Leonard B. Griffiths, North Reading; Vernon C. Potter, West Concord,Mass. [211 App]. No. 793,398 22 1 Filed Jan. 23, 1969 [45] Patented Dec.22, 1970 [73] Assignee P.R. Mallory & Co., Inc.

Indianapolis, Ind. a corporation of Delaware [54] PROCEDURE FOR CHILLCASTING BERYLLIUM COMPOSITE 12 Claims, 1 Drawing Fig.

[52] 0.8. CI. 164/68, 75/138; 164/61, 164/62: 75/150 I l .I,'.I Z11. T'1' f .f. [50] Field ot'Search 75/138, 150; 164/61, 68,62

[56] References Cited UNITED STATES PATENTS 3,049,421 8/ 1962 Allen eta1. 75/138X 3,337,334 8/1967 Fenn, Jr. et al. 75/150 2,399,104 4/1946Cooper 75/138 1,254,987 1/1918 Cooper 75/138 1,333,965 3/1920 Fahrenwald75/138 3,485,681 12/1969 Greenewald, Jr l64/322X ABSTRACT: A process forcasting aluminum-beryllium alloys comprising, charging a crucible withan aluminum base alloy placed at the bottom of a crucible, placingthereon a charge of beryllium in which the beryllium is in discretelumps or chunks, evacuating the furnace chamber for example to as low as150 microns to remove oxygen and other impurities from the atmosphere,raising the temperature of the charge sufficient to melt the aluminumbut insufficient to melt the beryllium, maintaining the aluminum moltenfor a time sufficient to degass the aluminum, changing the vacuum to anatmosphere of inert gas at a pressure below atmospheric, raising thetemperature of the charge sufficient to melt the beryllium, cooling thecharge to below the liquidus temperature for the particular Be-Al alloycomposition being cast, and rapidly pouring the charge into a mold madeof a material having high thermal conductivity and having a large mass,thereby rapidly cooling said charge.

ALUMlNUM BERYLLIUM PHASE DIAGRAM WEIGHT PER CENT BERYLLIUM s l0 I5 1300i TEMPERATURE, C

0 IO 20 3O 40 Al ATOMIC PER CENT BERYLLIUM PATENTEB DEEP-2 ALUMINUM-BERYLLIUM PHASE DIAGRAM WEIGHT PER CENT BERYLLIUM 0 e 5 B 4 6 H A m, m 0k w 0 O O 0 O O OO O O O O O O O O 0 w m m m w w mm 6 5 4 3 0 mama/512EATOMIC PER CENT BERYLLIUM sm m ROHR N mMWE R NHFT. O mmm A P m .G. J .CMBN AD u m mm% other temperatures and times could PROCEDURE FOR CHILLCASTING BERYLLIUM COMPOSITE Several processes for casting Be-Al alloyshave been reported comprising heating a charge of beryllium chunks andit aluminum alloy chinks in the presence of a fluxing agent selectedfrom the group consisting of alkali and alkaline earth halides to atemperature above the melting point of beryllium. The flux destroys theberyllium oxide film and enables the casting of a microstructure ofberyllium grains interspersed within an aluminum or aluminum alloymatrix.

However, under some casting conditions, particularly where theatmosphere has high humidity, it has been found that some porosity hasbeen observed in the castings.

Furthermore, it would be advantageous to avoid the flux because of itscorrosive nature, including its tendency to attack crucibles, the fumesit creates on volatilization and to avoid the cost outlay for the fluxmaterial.

Because of the corrosive nature of the flux, beryllium oxide cruciblesare preferred which are expensive.

Furthermore the microstructure obtained has not always been as fine asdesired and the aluminum matrix has sometimes contained undesirableimpurities.

It is therefore an object of the present invention to provide a processwhich avoids porosity in the castings.

It is another object of the present invention to provide a process whichavoids impurities in the aluminum matrix.

It is still another object of thepresent invention to provide a processwhich avoids the use of a halide flux.

It is still another object of the present invention to provide a processwhich permits the use of crucibles less expensive than beryllium oxidecrucibles.

It is still another object of the present invention to provide a processwhich results in a very fine as-cast microstructure.

Other objects will appear from the following description and drawing inwhich:

FIG. 1 is a view of the aluminum-beryllium binary phase diagram.

In order to achieve the foregoing objects the following procedure itutilized.

The mold to be used for casting is preheated in a furnace, for example,a temperature within the range of 700 F. to 800 F. for a period of timeof .5 to 1.5 hours. However, obviously be used. This step may beeliminated if desired.

After the preheating, the inside of the mold is coated with amorphouscarbon. This can be done for example with an acetylene torch adjustedtoa reducing flame or by spraying the mold with an amorphouscarbomThe-mold may then reheated for 20 to 30 minutes, for example.

The melting crucible, which may be made of MgO or other refractoryoxides rather than expensive BeO, is charged with beryllium in smallchunks. Preferably the size of the chunks is such that they will notpass through a one quarter mesh screen but will pass through a %inchesmesh screen.

The beryllium in the charge preferably has the followingcharacteristics. For nuclear applications the cobalt is preferred to bevery low, for example below 0.0005 percent. The iron content ispreferred to be below 0.1 percent. Other impurity elements should beless than 0.1 percent and the total of the other impurities should be 1percent.

The amount of Be in-the charge is preferably 68 to 85 per cent byweight. The remainder of the charge is aluminum or an alloy of aluminum.For example, one aluminum alloy which may be used contains 0.5 to 1.5percent magnesium and 0.5 to 1.5 silicon with the balance beingaluminum. The aluminum to be used is preferably 99.99 percent pure witha maximum con tent of impurities other than alloying elements of 0.01percent total. Furthermore, other alloying elements in addition tomagnesium and silicon may be desirablefor certain properties andcharacteristics of the material. It is to be understood that the processof the present invention is to include such additional alloying elementsin the aluminum matrix charge. While the matrix alloyhas been describedas an aluminum alloy, it

will be appreciated that some beryllium will end up in solid solutionwithin the aluminum matrix.

In charging the alloy the aluminum matrix alloy is placed at the bottomof the crucible and the beryllium chunks on top. The reason for thiswill become apparent hereinafter.

The preheated mold is placed in afurnace which has provisions forevacuation to as low as 150 microns and preferably to as low as microns,and below. Additionally, the furnace must have provisions for changingthe evacuated system to a system utilizing an inert gas preferably ofargon, but helium could also be used.

The melting crucible is placed on a rotatable axis inside the furnacewhereby the crucible can be rotated from outside the furnace and moltenmetal poured very rapidly into a mold which is situated below thecrucible. Alter placing the mold in the furnace below the rotatablecrucible, the furnace is evacuated to a pressure of below microns,preferably below 100 microns. The purpose of the evacuation is to removeoxygen and other impurities from the atmosphere. ln addition to removingoxygen and other impurities from the furnace atmosphere, the vacuumtreatment also degasses the aluminum matrix. Heat is gradually applied,for example by means of an induction coil. However, other types ofheating such as electrical resistance heating could also be used. Theheat is applied at a rate so that the aluminum is melted. After thealuminum is melted, the vacuum degasses the molten alloy. The berylliumchunks remain substantially unmelted. In the case where magnesium ispresent in the aluminum alloy, the magnesium causes sparking" or thevisual evolution of metallic particles from the partially melted charge.The beryllium charge must have sufiicient space between chunks so thatthere will be adequate space for these metallic particles and impuritiesin the molten aluminum to escape. The evolution of fsparking" marks thepoint wherein degassing has been completed. The sparking can be observedthrough a window in the furnace.

If desired, in order to observe the pressure at which degassing iscompleted, an alloy containing 0.5 to 1.5 percent magnesium in aluminum,together with beryllium chunks, may be processed through the furnace andthe pressure determined at which the sparking takes place. Then aluminumalloys not containing magnesium may be processed and the pressure atwhich sparking" occurred relied on to indicate when degassing has beensubstantially completed. For example, in one furnace a pressure of 450microns signifies the point where the degassing has taken place.However, it is apparent that the pressure at which degassing has takenplace will depend on the particular furnace and vacuum pump in which theoperation is being carried out.

After the degassing operation. has been concluded, the vacuum is shutoff and the chamber is back filled with an inert gas such as argon orhelium, preferably argon, to a pressure of at least 5 inches of mercurybelow atmospheric to prevent ad ditional magnesium evaporation and thefurnace temperature is raised in order to melt the beryllium. it can beobserved through the furnace window when all the beryllium chunks havemelted. In some cases if induction heating is used it may be observedthat an electric field created around the melt by the heating coil holdsthe beryllium chunks partially above the surface of the melt andtherefore the beryllium does not melt quickly. This can be avoided byturning the heating coil off so that there is no longer an upward forceacting on the beryllium and the beryllium can go down into the aluminumand be melted.

After all the beryllium has melted, the liquid is slowly cooled to thepouring temperature. The pouring temperature must be below the liquidustemperature shown on the Al-Be phase diagram for the particularaluminum-beryllium composition being used. It will be observed from FIG.1 that Be contents of 68-85 percent are on the Be side of the BeAleutectic point. For example the pouring temperature is 0 to 50 C. belowthe liquidus. As the temperature is lowered below the liquidus, athermal arrest or a point where no change in temperature with timeoccurs, is observed. Also the temperature may be observed from athermocouple placed in the melting crucible prior to pouring.

After the pouring temperature is reached, the charge is poured quicklyinto the mold in order to obtain turbulance. This can be done byrotating the crucible about its axis from outside the furnace. it isbelieved that this procedure results in a fine a microstructure becauseas the liquidus temperature is reached a certain number of berylliumnuclei are formed in the melt and begin to grow dendrites. In thecasting turbulence the dendrite arms break off and form new nucleationsites. It is therefore desirable to pour as rapidly as possible andstill avoid substantial amounts of metal splashing out of the mold. Thusa microstructure of fine Be grains in an aluminum matrix is obtained.

It is important that the casting is cooled very rapidly. The rate ofcooling is dependent upon the conductivity of the mold and the thicknessor mass of the mold. Materials having sufficiently high thermalconductivity to be used for the mold are as follows: copper, steel andgraphite or other carbonaceous materials. In the case where shapes whichare difficult to cast and which are subject to hot tearing are to becast, it is preferred to use graphite or other carbonaceous material forthe mold and for any cores which would be used.

The mold must also have adequate thickness or mass. It has been found,for example, that one and one quarter inch thick mold can be used toform castings 2 X 4 X 1/2 inch and a fine microstructure is obtained,but the thickness and mass will vary according to the shape and size ofthe casting.

In addition to rapid cooling, another embodiment of the inventioninvolves adding platinum to the melt in order to increase nucleation andobtain a fine grain structure. The platinum may be added in theelemental form with the charge or, alternatively, it may be added afterthe entire charge has been melted by the use of a strand which is guidedinto the melt from outside the furnace. The amount of platinum added isfor example from 0.05 to 0.5 percent by weight of the total chargepreferably 0.1 to 0.4 percent by weight. If platinum is added to thecharge, the pouring rate does not have to be quite so rapid as ifplatinum were not to be used.

We claim:

1. A method for casting aluminum-beryllium alloys comprising, charging acrucible with an aluminum base alloy placed at the bottom of a crucible,placing thereon a charge of beryllium in which the beryllium is indiscrete lumps or chunks, evacuating the furnace chamber, raising thetempera ture of the charge sufficient to melt the aluminum butinsufficient' to melt the beryllium, maintaining the aluminum molten fora time sufficient to Clegass the aluminum,"changing the vacuum to anatmosphere of inert gas at a pressure below atmospheric, raising thetemperaturerof the charge sufficient to melt the beryllium, cooling thecharge'to below the liquidus temperature for the particular'Be-Al alloycomposition being cast, and rapidly pouring the charge'into a mold madeof a material having high thermal conductivity and having a large mass,thereby rapidly cooling said charge.

2; The method according to claim 1 in which the beryllium content of thecharge is from 68 to 85 percent by weight beryllium.

3. A method according to claim 1 in which the aluminum charge containsmagnesium and during the degassing visible evolution of the particlesoccurs.

4. A method according to claim 1 in which the molten charge is cooled upto 50 50 C. below the liquidus prior to pouring. g

5. A method according to claim 3 in which the A1 charge also containssilicon.

6. A method according to claim 1 in which the mold is preheated prior tobeing placed in the melting and casting furnace.

7. A method according to claim 1 in which an addition of platinum isincluded in the molten charge prior to pouring to increase the rate ofnucleationl 8. A method according to claim 1 in which the furnace isevacuated at least as low as l50 microns.

9. A method according to claim 1 in which the inert has atmosphere is atleast 5 inches of mercury below atmospheric.

10. A method according to claim 8 in which the furnace is evacuated tobelow microns before the aluminum is melted.

11. A method according to claim 3 in. which inception os sparking isused to determine the point when the atmosphere of the furnace ischanged from a vacuum to an inert gas.

12. A method according to claim 1 in which the inert gas is selectedfrom the group consisting of argon and helium.

